Polymeric binders for electrophotographic coating applications



United States Patent 3,481,735 POLYMERIC BINDERS FOR ELECTROPHOTO-GRAPHIC COATING APPLICATIONS Richard B. Graver, Savage, and Stephen C.Heidecker and David D. Taft, Minneapolis, Minn., assignors, by mesneassignments, to Ashland Oil and Refining Company, a corporation ofKentucky No Drawing. Filed Feb. 23, 1966, Ser. No. 529,247 Int. Cl. G03g7/00, /00 U.S. Cl. 961.5 18 Claims ABSTRACT OF THE DISCLOSURE acid, and(3) a vinyl monomer free of either hydroxy or carboxy substituents aredisclosed as binders for use in electrophotographic coatingapplications.

The present invention relates to electrophotography. In one aspect, thepresent invention relates to the use of polymers of hydroxyalkyl estersof acrylic or methacrylic acid as binders for zinc oxide inmanufacturing coated copy paper. In still another aspect, the presentinvention relates to the use of such coated copy paper in directelectrophotographic processes.

Electrophotography is one of several terms used to describe areproduction or image transfer process utilizing electrical and lightstimulation of conductive materials. Some common terms for commercialprocesses based on this technique are Electrofax, electrostatography,electrography, xerography, and elcctrographic recording. These processesall employ electromagnetic radiation and a photo-responsive member toobtain, on exposure to light, a latent electrostatic image on thephoto-responsive member. Ordinarily, this latent electrostatic image isthen converted into a positive image, off-set master, etc. Such techniques areused in commercial off-set printing, computer readout, filmdeveloping, map making, long-distance copying, recording, oflicecopying, and the like.

There are two general types of electrophotography which'have beencommercially successful in the office and industrial copying field.These types, although using the same basic principle, differ in the typeof photoresponsive member employed. These types of electrophotographyare the direct process and the transfer process.

THE TRANSFER PROCESS The transfer process relies upon thephotoconductive properties of amorphous selenium. This process operatesin the following manner:

(1) A selenium-coated aluminum drum is given a uniform electrostaticcharge by exposure to a corona discharge in the dark.

(2) A light image of the object to be copied (i.e. the object image) isthen projected through a lens system in such a manner that the lightstrikes the charged drum, dissipating the previously-applied charge inthe nonimaged area. As a result, the drum then carries an electricalcharge in those areas corresponding to, for example, printed mattercontained on the object being copied (e.g. a page of a book).

(3) The charged latent image is then developed by cascading anoppositely charged dry toner (or ink) across the surface of the drum.The powdered toner clings to the oppositely charged areas on the drum byelectrostatic attraction. In this manner, a powder or toner image of theobject being copied is formed on the selenium-coated drum.

ice

(4) Finally, a sheet of ordinary paper (i.e. paper not coated with aphotoconductor) is placed over the powder image and given anelectrostatic charge by the use of a corona discharge. The chargedpowder or toner is then transferred from the drum to the paper.

(5) The paper is then heated to, for example, 200 C. to fuse the powderon the surface of the paper and produce the fixed image.

THE DIRECT PROCESS The direct process utilizes the same principle as thetransfer process, except that a selenium-coated drum is not used.Instead, the image is directly produced on, for example, paper. Thebasis for this technique is the use of coated paper or the likecontaining, on the surface thereof, a finely-divided photoconductivematerial. Typically, zinc oxide is employed as the photoconductivematerial and is bonded to the paper 'by the use of some suitable organicbinder. Often, dyes are used to sensitize the zinc oxide. The electricalconductivity of zinc oxide shows a significant increase when subjectedto light. Consequently, the zinc oxide will lose any previously appliedelectrical charge when exposed to light.

The direct process operates as follows:

(1) The coated paper is made light sensitive by placing a uniformelectrostatic charge on the coated surface in the dark by means of ahigh voltage corona discharge, e.g. 4000 to 6000 volts. The coated paperis not sensitive to light until properly charged.

(2) A latent image is produced on the paper by projecting an objectimage onto the paper, typically employing white light in the visiblerange. The original electrostatic charge carried by the paper isdissipated in all areas exposed to light and is retained in the shadowedareas, thus forming a latent electrical image on the paper.

3) Development of the latent image is similar to that described for thetransfer process, except that commercial equipment is available whichutilizes both liquid and dry toners. In the dry method, the toner(consisting of pigmented resin and ferromagnetic particles, e.g. ironfilings) is applied to the paper by a magnetic brush. As the brush movesacross the paper, the charged toner is attracted to the charges ofopposite polarity on the paper. In the liquid method, the pigmentedresin particles are suspended in an organic liquid such as odorlessmineral spirits. This liquid is then brought in contact with the paper.

(4) Two methods are utilized to permanently fix the image to the paper.With the dry toner system, the image is fixed by heat (below thecharring temperature of the paper) which fuses the toner to the coatedpaper. Fixing the image in the liquid toner system is accomplished byheat and solvent evaporation.

Multi-color copying can be conducted by re-charging the coated copypaper for each new color and utilizing the proper color of toner. Thistechnique is currently applied in map copying.

The binder for the zinc oxide (or other photoconductor) is extremelyimportant to the success of the electrophotographic process. The bindershould be one into which the zinc oxide (or other photoconductor) can bedispersed and-the mixture then applied to paper as a coating. The bindermust not interfere with the photoconductive properties of the zincoxide. The binder should adhere strongly to the paper and provide aflexible coating. Since the binder is ordinarily applied to a sheet ofpaper, it should be able to withstand 180 bends and wrinkling withoutcracking or chipping. The binder, when mixed with zinc oxide and appliedto a paper substrate, should be capable of providing pleasing, clear,legible copies. To enable the photoconductor to accept a maximumelectrostatic charge, it is desirable that the individual photoconductorparticles be separately encapsulated by the binder.

For this latter purpose, the binder should be capable of properlywetting the photoconductor and firmly positioning it on the paper in thephotoconductive matrix. Initially, the binder must be an extremely goodinsulator and prevent any significant decay of the electrostatic chargeon the paper (before the photoconductive coating is exposed to the lightof the object image). Additionally, the binder should not interfere withthe rapid dissipation of the electrostatic charge when the coated paperis ex osed to the light of the object image. Desirably, the bindershould not accept or retain any appreciable residual voltage in theexposed area. The binder should not exhibit color or decompose on aging.The binder should bond readily to the pigmented developer or toner andgive rise to good printing quality. Desirably, the binder should beeffective when used at very low coating weights. Additional propertieswhich the binder should exhibit are nonyellowing tendencies, resistanceto solvents (if the coated paper is to be used in combination with aliquid toner) and a resistance to flowing at the fixing or fusingtemperature of the toner. Still further, the electrical properties ofthe coated paper should not be significantly affected by changes inhumidity.

At the present time, a variety of binders have been suggested for use inmanufacturing paper for direct electrophotography. In certain instances,some acrylic resins have been utilized as modifiers for the moreconventional binders. However, acrylic resins have not been used to anysignificant extent as the primary or sole binders because of certainundesirable properties which they exhibit. For example, in the wet tonerprocess, the use of high molecular weight acrylic resins has increasedwhat is known as solvent holdout. The increased solvent holdout did notallow a suitable fusing of pigment particles to the surface of theexposed paper. Consequently, the pigmented particles did not adhere andsmeared. The resulting copies were of poor quality.

It has now been discovered that a specific type of acrylic copolymergives outstanding physical and electrical properties when used as abinder, even in the wet toner process. These copolymers contain (1) from1 to 30% by weight (preferably 10 to 20% by weight) of a hydroxyalkylacrylate or methacrylate, or mixture thereof, (2) from 1 to 15% byweight (preferably 1 to 5% by weight) of a copolymerizablea,/3-unsaturated carboxylic acid, or mixture thereof, and (3) thebalance to make 100% of one or more copolymerizable monoethylenicallyunsaturated vinyl monomers which are devoid of hydroxyl groups. Thecopolymers can be prepared from these monomers by conventionaltechniques as shown in, for example, US. Patents 2,681,897, 3,082,184,and 3,198,850. The copolymers can be used as binders both with andwithout cross-linking agents such as the various aminoplasts (e.g. anmelamine-formaldehyde resin) and phenolis (e.g. phenol-formaldehyderesins).

One of the classes of monomers used in preparing copolymers for use inthis invention consists of the hydroxyalkyl esters of vinyl carboxylicacids. Suitable hy droxyl-containing esters are the C C hydroxyalkylesters of acrylic and methacrylic acids, as well as mixtures thereof.Typically, the hydroxyl groups will be in the beta (i.e. 2), gamma (i.e.3), etc., position. Ordinarily, beta-hydroxyalkyl esters of acrylic ormethacrylic acids will be used. Suitable hydroxyalkyl esters arefl-hydroxyethyl acrylate and methacrylate, fi-hydroxypropyl acrylate andmethacrylate, ,B-hydroxyhexyl acrylate and methacrylate, fi-hydroxydecylacrylate and methacrylate, 12- hydroxystearyl acrylate and methacrylate,and the like. Hydroxypropyl acrylate and methacrylate, as well asmixtures thereof, are the preferred hydroxyalkyl esters. Hydroxypropylmethacrylate is especially preferred.

A second class of monomers used in preparing the copolymers consists ofthe copolymerizable carboxylic acids. Any copolymerizable unsaturatedcarboxylic acid can be used in preparing copolymers for use in thepresent invention. Suitable acids include maleic acid, fumaric acid,itaconic acid, citraconic acid, crotonic acid, acrylic acid, methacrylicacid, and the like. Acrylic acid, methacrylic acid, and itaco nic acidare preferred. If desired, mixtures of these acids can be used. Often,certain of these acids are present in commercially availablehydroxyalkyl esters. Thus, a single commercial raw material can providetwo of the three classes of monomers needed to form the copolymers usedin this invention.

A third class of monomers used in preparing the copolymers consists ofother mono-ethylenically unsaturated compounds which are free ofhydroxyl and carboxyl groups and which are copolymerizable with thefirst two monomer classes hereinbefore mentioned. Othermonoethylenically unsaturated compounds copolymerizable with thehydroxyl-containing and acidic monomers are the C C cyclic and acyclicesters of acrylic and methacrylic acids, acrylonitrile,methacrylonitrile, styrene, ochlorostyrene, vinyl toluene, a-methylstyrene, etc. If desired, mixtures of these vinyl monomers can be used.The hardness, flexibility, and adhesion of the copolymers to varioussubstrates can be varied by adjusting the proportions of these variousvinyl monomers. Styrene and the C -C alkyl acrylates and methacrylates,as well as mixtures thereof, are preferred.

For coating paper, the copolymers of this invention can be optionallycombined with aminoplast or phenolic resins. Suitable aminoplast resinsinclude the alkylated and non-alkylated condensates of an aldehyde withurea, N,N-ethylene urea, dicyandiamide, and aminotriazines. If watersoluble condensates are preferred, as in combination with an emulsion orwater-soluble copolymer, the non-alkylated or partially alkylated watersoluble aminoplasts are preferred over the more fully alkylatedaminoplasts. If a solution copolymer is utilized, alkylated aminoplastssoluble in the organic solvent of the copolymer are preferred.Phenol-formaldehyde resins can also be used. The amount of aminoplast orphenolic resin used in conjunction with the copolymers will usually befrom 5-100%, e.g., 1050%, based on the weight of the copolymer.

In preparing coating or binder compositions for application to asuitable substrate (e.g., paper) a photoconductive material is dispersedin a solution or emulsion of the copolymer by suitable grindingtechniques. Typical photoconductive materials are zinc oxide, zincsulfide, silver chloride, mercuric sulfide, and other photoconductivematerials known to the art. Zinc oxide is the preferred photoconductor.The zinc oxide is usually dispersed at a concentration of 4-16 parts byWeight of zinc oxide per one part of copolymer. This mixture isoptionally combined with the aminoplast or phenolic resin. Preferably,the zinc oxide is dispersed or mixed at a level of 6 to 12 parts byweight of zinc oxide per one part of copolymer, and more preferably 8-10parts by weight of zinc oxide per one part of binder.

If an emulsion copolymer is utilized as the binder, the zinc oxide canfirst be dispersed in a suitable dispersing aid. Then, this dispersantmixture can be diluted with the copolymeric emulsion in a mannerconventional to the art of pigment dispersion.

In any case, the final binder or coating compositions will usually havea solids content (i.e., non-volatile content) of from 4070%, preferably50-60% (the remainder being solvent or water).

In order to render the dry, cured photoconductive coatings morereceptive to a charge and a photoconductive effect, suitable dyes suchas rose bengal, methylene blue, rhodamine B, dibromo-fluorescein, andvarious cyanine dyes can be included in the binder or coatingcompositions. Usually a mixture of dyes is added, incorporatingcomplementary colors so that the dried coatings have a pleasing,off-white appearance. These techniques and others similar to the art ofelectrophotographic binder compositions can be applied to the coating orbinder compositions of this invention.

The coating compositions, containing the zinc oxide, the copolymer,solvent or water, and optionally an aminoplast or phenolic resin, arethen applied to a suitable substrate (usually paper, although metal,foil, etc., can be used) in any suitable fashion such as by brushing,spraying, dipping, roller coating, or the like to give a coated paperhaving an average of from 5-50 pounds, e.g., 8-30 pounds of depositeddry coating per 3000 square feet of coated paper surface. The wet coatedpaper can be air dried at room temperature or dried by baking, e.g.,baked at 200350 F. The preferred drying temperature will depend onwhether or not an aminoplast or phenolic resin is present. Without anaminoplast or phenolic resin, a curing temperature in the range of150250 F., preferably l90210 F. will ordinarily be used. In the presenceof an aminoplast or phenolic resin, higher temperatures are usuallyrequired to obtain maximum benefits. The addition of small portions (0.1to 1%) of an acidic catalyst such as toluene sulfonic acid or phthalicacid serves to lower the temperature required for proper curing.

The present invention will be further understood by reference to thefollowing specific examples which include a preferred embodiment. Unlessotherwise indicated, all parts and percentages are by weight.

Example 1 16 parts of methyl methacrylate, 26.6 parts of styrene, 19.9parts of butyl methacrylate, 19.9 parts of butyl acrylate, and 37.4parts of a commercially available xylene solution containing 40% ofhydroxypropyl methacrylate and 6% of methacrylic acid were mixedtogether. A mixture of 1.25 parts of azo-bis-isobutyronitrile and 0.2part of di-tertiary butyl peroxide was then added to the mixture ofmonomers. The resulting mixture was then sparged with nitrogen and thenadded dropwise during a 2 /2 hour period at 250 F. to a solution ofxylene and n-butanol contained in a 2-liter, three-necked, glass flaskequipped with an agitator, condenser, nitrogen inlet, thermometer, andaddition tube. During this addition, the xylene/n-butanol solution(which had been carefully sparged with nitrogen) was agitated. Includingthe time required for addition, the reaction mixture was heated at 250F. for 8% hours. The resulting mixture was then cooled. This mixture hada viscosity of 6 stokes and an acid value of 12 at a non-volatilecontent of 49.9%. Of the 50.1% volatile (i.e., solvent) portion of thismixture, 90% was xylene and 10% was n-butanol.

A 250 gram portion of zinc oxide (American Zinc Sales AZOZZZ661) wasthen added to 50 grams of the copolymer solution just prepared to give aweight ratio of zinc oxide to copolymer of about 10: 1. The zincoxidewas dispersed in the solution by agitation for 5 minutes in aHamilton-Beach mixer. The dispersion was then diluted with toluene to a58% concentration of pigment and resin (i.e., 42% solvent).

Next, the following mixture of dyes was added to sensitize the zincoxide/copolymer composition:

0.086 gram of a 1% solution in methanol of Hidacid Eosine (Hilton-Davis)0.986 gram of sodium fluorescein (1% solution in methanol) 0.357 gram ofAlphazurine 2G (Allied Chemical) (1% solution in methanol) 0.571 gram ofAlizarine Cyanine Green GWA (Gen.

Aniline & Film) (1% solution in methanol) The resulting coating orbinder composition was then applied to paper at a rate sufficient togive 22, 15, 10, and 8 lb. of dry coating/ 3000 sq. feet, using theappropriate coating rod for this purpose. The wet, coated sheets werebaked at 120 F. for 5 minutes. Then the dry, tack-free coated sheetswere allowed to stand in the dark for 24 hours at a humidity of 50% at77 F.

TABLE I Light Charge Dark decay Residual Dry Coating Wt. acceptance,deca time, charge, (lbs/3000 sq. ft.) volts volts seconds volts Inaddition to giving excellent image quality, the copy paper (i.e. thecoated sheets) exhibited excellent adhesion and outstanding marresistance. The coated sheets also displayed unusual pre-foggingproperties in that light exposure immediately prior to charge acceptancedid not affect the amount of charge acceptance.

Most important was the ability of the coated sheets to provide copies ofexcellent contrast, image density, and spectral response especially atthe low coating weights. This high quality copy was obtained, despitethe relatively low charge acceptance of the coated sheet.

Examples 2 and 3 In Example 2, a mixture of 177.6 grams of methylmethacrylate, 201.6 grams of styrene, 272.4 grams of butyl methacrylate,272.4 grams of butyl acrylate, 97.2 grams of acrylic acid, and 147.5grams of a 96% solution of hydroxypropyl methacrylate in xylene (whichalso contained 4.8 grams of methacrylic acid), 24 grams ofazobis-isobutyronitrile and 12 grams of di-tertiarybutyl peroxide wasadded to 340 grams of ethyl Cellosolve in an autoclave. Themonomer/catalyst mixture was added over a two-hour period to theautoclave which was maintained at a temperature of 345 350 F. Thepressure increased to 100 p.s.i.g. during the addition. Additionalsolvent (122 grams) was added to obtain a 70% NV solution. A boostercatalyst of 3.5 grams of di-tertiarybutyl peroxide was added four hoursafter the monomer addition was completed. The total reaction time(including monomer addition) was ten hours. The copolymer had a finalviscosity of 14 stokes at 70.2% NV. The acid value of the solution was45.6. An 800 gram portion of this copolymeric solution was mixed with239 grams of ethyl Cellosolve, 41.5 grams of triethyl amine, and 280.5grams of water. The resulting copolymeric dispersion had a viscosity of4100 cps. (Brookfield RVT viscometer using a #4 spindle at .20 r.p.m.)and a pH of 7.9. The dispersion was 52.5%

In Example 3, a similar copolymer was prepared except that the amount ofacrylic acid was greater: 114 grams instead of 97.2 grams as in Example2. The rest of the copolymer composition and the procedure wereidentical. The final copolymeric solution had a non-volatile content of69.8%, a viscosity of 14.8 stokes, and an acid value of 51.7. A 468 gramportion of this copolymeric resin was mixed with 202 grams of ethylCellosolve, 39.4 grams triethyl amine, and 226.6 grams of water. Theresulting resin dispersion had a non-volatile content of 52.3, aviscosity of 1920 cps. (#4 spindle at 100 r.p.m.), and a pH=7.9.

Next, 50 gram portions of the copolymeric dispersions of Examples 2 and3 were mixed with zinc oxide and sensitizing dyes in the mannerdescribed in Example 1.

' The weight ratio of zinc oxide to copolymer was about 7 of drycoating/3000 sq. ft. of paper. The following electrical results wereobtained:

TABLE II Dispersion of Example 2 (grams).

8 Examples 4-11 A number of additional copolymers were prepared in themanner described in Example 1. Zinc oxide was dispersed in each of thesecopolymers in the manner de- Dispersion of Example 3 (grams) 5 scribedin Example 1. The amount of zinc oxide was suffi- Zinc oxide (grams) 250250 Charge acceptance 070m) clent to give a weight ratio of Zinc oxideto copolymer of (a) At 221b./3,000 sq. ft 440 460 about 1. A dyemixture, equivalent to that of Example (b) At lb./3,000 sq. ft. 360 3701 dd 1 Dark Decaywoltsk was a ed to the 21m: oxide/copo ymer mixture toAt 15/ 9 q-f 28 :3 give 100 p.p.m. of dye based on the total we1ght ofzinc 10 oxide. The resulting coating compositions were then used At22lb./3,000 sq. ft 3% 4% to coat a er at a wei ht of 22 ounds of drcoatin er (1)) At 15 lb./3,000 sq. it 3% 4% p p g p Y g pResidualvoltage (volts). 3000 sq. ft. of paper surface. The formulatlonof these t qcopolymers and their correspondin" properties are shown 15.3 000 20 20 a (b) At 113/ Sq ft 1n Table HI which follows:

TABLE III Example Copolymer composition:

n-Butyl acrylate 20.0 20.5 20.5 18.2 20.5 18 2 23.5 24.1 n-Butylmethacrylate .1 20.0 20.5 20.5 18.2 20.5 18 2 23.5 24.1 Styrene. .3Methyl methacrylate. 1 9 Methacrylic acid 6 Hydroxypropyl acrylate 5Hydroxypropyl methacrylate D i-tertiarybutyl peroxide...

Azo-bis-butyronitn'le Solvent composition, percent:

Butanol.---..

Physical properties of the copolymerie solutions:

N on-volatile. Acid value (100% NV)..- Viscosity (stokes at 0.).Hydroxyl value (100% NV) Electrical properties of the coated paperCharge acceptance (volts) 2 Dark decay (volts) 3 Light decay time(seconds) Residual voltage (volts) 5 1 The hydroxypropyl methacrylatewas supplied as a solution from Rohm and Haas Company. It is known to bea mixture of isomeric hydroxyalkyl acrylates. The solution contains38.5-42.5% hydroxypropyl methacrylate, a maximum of 2.0% of highermethacrylates, a maximum of 0.3% of alkylene di-methacrylate, and about56% of methacrylic acid.

2 Charge acceptance is the voltage from the base line to the maximumabsorption of voltage.

3 Dark decay is the voltage drop in darkness over a 4.5 second periodfrom the maximum charge acceptance to the start of the light decay.

4 Light decay is the time required for the static charge (i.e. theaccepted charge) to be dissipated to 50 volts.

5 The residual voltage is the amount of static charge which is notdissipated.

EXAMPLES 12-22 TABLE IV Example copolymer composition:

n-Butyl acrylate 19.9 19.9 19.9 19.9 28.3 29.9 19.8 19.7

n-Butyl methacrylate-. 19 9 19.9 19.9 19.9 .7

t-Butyl methacrylate-. 14.9

2-ethyl hexyl acrylate.

Methyl methacrylate Methacrylic acid.-.

Styrene 0zMethyl styrene Hydroxypropyl methacrylate.

Hydroxypropyl acrylate Azo-bis-butyronitrile Solvent composition,percent ol Plti ysical properties of the copolymeric solu- N on-volatileAcid value (100% NV). Viscosity (stokes at 25 C Hydroxyl value (100% NV)Electrical properties of the coated Charge acceptance (volts) Dark decay(volts).- Light decay time (sec Residual voltage (volts) 15. 8 8. 3 l5.2 16. 5 9. 4 9. 3 9. 4 l1. 8 11. 2 l0. 5 13. 8 22 5. 5 13. 5 8 5. 0 19.4 5. 6 15. 0 13. 1 6. 4 l0. 9

In addition to the excellent results obtained with the use of theseparticular acrylic copolymers, it has been noted that the additional useof large amounts of cobalt naphthenate (e.g. 0.15% to 0.5% based on theweight of the copolymers) increases the charge acceptance and reducesthe light decay time. While cobalt naphthenate is sometimes used as adrier for oxidizing resins at levels of, for example, 0.01 weightpercent, its use at high levels in conjunction with a non-oxidizingcopolymer is very unusual. Other known driers, e.g. lead naphthenate, donot exhibit the same degree of effectiveness in non-oxidizing resins.

Although the present invention has been described with a certain degreeof particularity, it is not intended that the invention be limited tothe specific materials and specific proportions which have been givenfor the sake of illustration. Numerous modifications and variations willappear obvious to one skilled in the art.

What is claimed is:

1. In the method of image reproduction by an electrophotographic processwherein a latent electrostatic image is produced on a substrate having afinely divided photoconductive material bound to the surface thereofwith an organic binder, the improvement which comprises using as saidbinder a copolymer of:

(a) from 1 to 30% by weight of a hydroxyalkyl ester of a vinylcarboxylic acid, or a mixture thereof;

(b) from 1 to 15% by weight of a copolymerizable a,,8-unsaturatedcarboxylic acid, or a mixture thereof, and

(c) the balance to make 100% of at least one copolymerizablehydroxyl-free, carboxyl-free mono-ethylenically unsaturated vinylmonomer.

2. The method of claim 1 wherein said hydroxyalkyl ester is a C -Chydroxyalkyl acrylate or methacrylate, or a mixture thereof.

3. The method of claim 2 wherein said n e-unsaturated carboxylic acid isacrylic acid, methacrylic acid, itaconic acid, or a mixture thereof.

4. The method of claim 3 wherein said hydroxyl-free vinyl monomer isstyrene or a C -C alkyl acrylate or methacrylate, or a mixture thereof.

5. The method of claim 4 wherein said photoconductive material is zincoxide and the weight ratio of zinc oxide to said copolymer is from 4:1to 16: 1.

6. The method of claim 5 wherein said substrate is paper.

7. The method of claim 6 wherein an aminoplast resin is used with saidcopolymer to bind said zinc oxide to the paper.

8. The method of claim 6 wherein said copolymer is the only binderpresent.

9. In the method of image reproduction by an electrophotographic processwherein a latent electrostatic image is produced on paper having finelydivided zinc oxide bound to the surface thereof with an organic binder,the improvement which comprises using as said binder a copolymer of:

(a) from 10-20% by weight of a C -C hydroxyalkyl acrylate ormethacrylate, or a mixture thereof;

(b) from 1-5% by weight of acrylic acid, methacrylic acid, maleic acid,fumaric acid, itaconic acid, citraconic acid, crotonic acid, or amixture thereof; and

(c) the balance to make 100% of C C cyclic or acyclic ester of acrylicor methacrylic acids, acrylonitrile, methacrylonitrile, styrene,o-chlorostyrene, vinyl toluene, a-methyl styrene, or a mixture thereof.

10. The method of claim 9 wherein (b) is acrylic acid, methacrylic acid,itaconic acid, or a mixture thereof.

11. The method of claim 10 wherein (c) is styrene C C alkyl acrylate ormethacrylate, or a mixture thereof.

12. The method of claim 11 wherein (a) comprises hydroxypropyl acrylateor methacrylate, or a mixture thereof. I

13. The method of claim 12 wherein the weight ratio of zinc oxide tosaid copolymer is from 6:1 to 12:1.

14. As a binder composition, a mixture consisting essentially of:

(a) a copolymer of (1) from 1 to 30% by weight of a hydroxyalkyl esterof a vinyl carboxylic acid, or a mixture thereof; (2) from 1 to 15% byweight of a' copolymerizable a,,8-unsaturated carboxylic acid, or amiXture thereof, and (3) the balance to make 100% of at least onecopolymerizable hydroxyl-free, carboXyl-free mouo-ethylenicallyunsaturated vinyl monomer;

(b) finely divided zinc oxide in a weight ratio of zinc oxide to saidcopolymer of from 4:1 to 16:1; and

(c) as a diluent, solvent or water, or a mixture there- 15. Thecomposition of claim 14 wherein the nonvolatile content of said mixtureis 40 to weight percent.

16. The composition of claim 15 wherein said diluent is a mixture ofsolvent and water.

17. The composition of claim 14 wherein said mixture also containssensitizing dyes for said zinc oxide and contains from 10-50 weightpercent aminoplast resin based on the weight of said copolymer.

18. Coated copy paper comprising paper having a cured coating on asurface thereof, said coating having been formed by applying to saidpaper a wet film of the binder composition of claim 14 and thereaftercuring said composition on said paper, said coated copy aper having anaverage of from 5-50 pounds of deposited dry coating per 3000 squarefeet of coated paper surface.

References Cited UNITED STATES PATENTS 2,681,897 6/1954 Frazier et al260-850 3,245,786 4/1966 Cassiers et al 96-1.8 3,331,687 7/1967 Kosche961.5

NORMAN G. TORCHIN, Primary Examiner I. R. HIGHTOWER, Assistant ExaminerUS. Cl. X.R. 96--1.8; 252-501

